1. Field of the Invention
The present invention relates to an evaporation dehydrator or treater, designed to vapourize and break out contained water from a pre-heated heavy oil emulsion feedstock by a sequential combination of evaporation, defoaming, and vapour-liquid separation steps. The treater is further adapted to hold the hot product under quiescent conditions in a sump zone for a pre-determined retention period, to settle out contained solids. The invention further relates to a method, comprising: pre-heating the feedstock to at least the boiling temperature of water, breaking out part of the contained water, defoaming the stream, heating the stream as it flows over a flowpath formed by a zigzag array of heated surfaces, to evaporate retained water, and retaining the dewatered product under quiescent conditions for a period of time, to settle out contained solids.
The apparatus and method find particular application in the treatment of thermal project production well streams.
2. Background
As stated, the invention finds application in the treatment of production streams produced from heavy oil reservoirs and in particular those using thermal recovery techniques.
Thermal recovery techniques, such as in situ combustion, steam injection, and cyclic steam injection and blowdown, are commonly used to produce heavy oil reservoirs. These procedures, as is well understood, are designed to reduce the viscosity of the immobile oil and improve its mobility to such an extert that it can be driven to a production well and recovered.
The production well streams from such thermal projects commonly comprise oil, water, gases and solids, usually tightly bound together in the form of an emulsion. The compositions can vary widely. In the specific instance of water content, it commonly is as high as 10% by volume.
In this state, the production is not saleable to pipeline operators. They commonly specify that the basic sediments plus water (BS&W) content must be less than 0.5% by volume.
As a result, thermal projection production must normally be "cleaned up" by water and solids removal.
This is attempted, in most cases, by heating the production and retaining it in a vessel to settle the water and solids. The cleaned oil is decanted and the water and solids are withdrawn from the base of the tank. To improve the operation, it is common to mix a chemical demulsifier with the production, prior to introducing it into the vessel. This operation is commonly referred to as a gravity settling treatment system.
When this type of system is relied on to clean up heavy oil thermal project production, the results are frequently poor. There are several reasons one can point to. Firstly, the density of the oil is so close to that of the contained water, that separation by settling is very slow. The high viscosity of the oil is another factor which interferes with settling. And finally, the emulsions which are involved are very stable, having been stabilized by fine solids and having been subjected to agitation by the gases co-produced with the oil.
As a case in point, the present invention was first used in conjunction with a steam drive project located at Lindbergh, Alberta. This project had to be shut down, because the conventional gravity settling treatment system used could not be operated to reduce the BS&W content below 1-5%, even though the production was repeatedly passed through the system.
The performance of a gravity settling treatment system can be significantly improved if a light hydrocarbon diluent is added to the emulsion, to increase the density differential between the oil and water. However, the diluent usually is not present on the project site and has to be brought in, which is expensive.
There is thus a need for a system adapted to clean up thermal project production to a BS&W content which is consistently below the maximum 0.5% specification. It would be particularly useful if such system were to be adapted to yield light hydrocarbons as a by-product, for use as a diluent in the free water knockout circuit used in conjunction therewith.
Turning now to the patent literature, there are several patents of which we are aware that are of interest. These are: U.S. Pat. Nos. 40,662 issued to Miller; 1,070,555 issued to Stone; 1,443,743 issued to Hess; and 1,528,968 issued to Brown.